CAS is a gene that encodes a protein that is localized to microtubules and to the mitotic spindle. CAS is the mammalian homologue of CSE1, a yeast gene which controls mitosis. CAS may have a similar role in mammalian cells. Because CAS expression is high in proliferating cells, anti-CAS antibodies have been used as a proliferation marker in leukemia and lymphoma cells. We have identified different spliced forms of CAS mRNA which may confer different functions in these tissues. To help clarify the role of CAS, the mouse homologue has been isolated and has been used to construct knock out mice. The mice are embryonic lethals. Heterozygous mice are being crossed with transgenic mice with incidence of cancers increased to investigate the role of CAS in tumorogenesis